In this image made from video provided by NASA, astronauts Chris Cassidy, foreground, and Tom Marshburn prepare on Friday for a spacewalk from the International Space Station. / AP

by USA TODAY

by USA TODAY

WASHINGTON (AP) - Two astronauts will make a precedent-setting spacewalk Saturday to try to fix an ammonia leak in the International Space Station's power system. Spacewalks are rarely done on such short notice, but the U.S. space agency says the six-member crew is not in danger.

The ammonia leak was discovered Thursday and forced the shutdown of one of eight solar panels that power the station, but the outpost can operate fine with only seven, NASA spokesman Kelly Humphries said.

One of the spacewalk veterans scheduled for the job was due to return to Earth on Monday, one reason NASA wants to tackle the problem this weekend, Humphries said.

Station Commander Chris Hadfield of Canada told NASA flight controllers Friday that the crew is completely ready for the spacewalk.

"I think it's really smart the way we're all proceeding here," Hadfield radioed down to Earth. "It's the right thing to do."

Hadfield tweeted that the crew was working "like clockwork" and said the two U.S. astronauts, Chris Cassidy and Tom Marshburn, were already getting their spacesuits ready, adding "Cool!"

The leak is in one of the radiator lines that chill the power systems. NASA spokesman Rob Navias said the line was expected to run out of ammonia coolant Friday. Power has been rerouted and is operating normally, he said.

The repair is what NASA calls one of the "Big 12" types of emergency repair work that all spacewalking astronauts train for in advance, Navias said.

NASA suspects the leak might be on the far left truss of the station from a pump box, which will be swapped out with a nearby spare during the spacewalk.

In 2009, Cassidy and Marshburn flew to the space station on the shuttle Endeavour and walked in space together to swap out a battery in the same location, so "they know this work site inside and out," Navias said.

Marshburn, Hadfield and Russia's Roman Romanenko are set to return to Earth on Monday. NASA Administrator Charles Bolden told reporters on Friday that their return will go ahead as planned.

Another reason to do the repair quickly: There may be some ammonia left which will help the spacewalkers find the leak, which is generating visible white flakes, Humphries said.

Last year, station instruments revealed a radiator leak that was so small that it wasn't visible. It was in the same general area, but NASA isn't sure if it is the same leak or not, he said.

In November, two other spacewalking astronauts tried to reroute coolant lines to bypass the tiny leak but it wasn't successful, he said.